1. Technical Field
The present invention relates to a honeycomb structure used in, for example, a catalyst carrier utilizing a catalytic action, for use in an internal combustion engine, a boiler, a chemical reactor, a fuel cell reformer, etc., a filter for trapping fine particles present in an exhaust gas, and to an assembly thereof. More particularly, the present invention relates to a honeycomb structure that is resistant to breaks when exposed to a thermal stress during its use, as well as to an assembly thereof.
2. Background Art
Honeycomb structures are in use in, for example, a carrier for a catalyst having a catalytic action, for use in an internal combustion engine, a boiler, a chemical reactor, a fuel cell reformer, etc., and a filter for trapping fine particles present in an exhaust gas, particularly fine particles emitted from a diesel engine.
In a honeycomb structure used for such a purpose, a sharp temperature change of exhaust gas and local heating make non-uniform the temperature distribution inside the honeycomb structure, and there have been problems such as crack generation in the honeycomb structure and the like. When the honeycomb structure is used particularly as a filter for trapping a particulate matter in an exhaust gas emitted from a diesel engine, it is necessary to burn the fine carbon particles deposited on the filter to remove the particles and regenerate the filter and, in that case, high temperatures are inevitably generated locally in the filter; as a result, large thermal stress and cracks tend to be generated.
Hence, there have been proposed processes for producing a honeycomb structure by bonding a plurality of individual segments using an adhesive. For example, U.S. Pat. No. 4,335,783 discloses a process for producing a honeycomb structure that comprises bonding a large number of honeycomb parts using a discontinuous adhesive. Also, JP-B-61-51240 proposes a thermal-shock resistant rotary regenerative heat exchanger that is formed by extrusion molding a matrix segment of honeycomb structure made of a ceramic material; cutting the structure into segments; firing them; making smooth, by processing, the outer peripheral portion of a fired segment; coating the part subject to bonding of the resulting segment with a ceramic adhesive which turns, after firing, to have substantially the same chemical composition as the matrix segment and a difference in thermal expansion coefficient of 0.1% or less at 800° C.; and firing the coated segments. Also, SAE paper 860008 (1986) discloses a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with a cordierite cement. Further, JP-A-8-28246 discloses a ceramic honeycomb structure obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles.
Meanwhile, regulations for exhaust gas have become stricter and engines have come to have higher performances. As a result, in order to achieve an improvement in combustion conditions of an engine and an increase in purification ability of a catalyst, the temperature of exhaust gas has increased year by year. In this connection, a higher thermal shock resistance has come to be required for the honeycomb substrate. Therefore, even with honeycomb structures such as mentioned above, when a sharp temperature change of inflow gas takes place, and a local heat of reaction, a local heat of combustion, etc., becomes larger during use, it is possible that the thermal stress applied thereto may not be sufficiently relaxed, cracks may appear therein and, in an extreme case, for example, the honeycomb structure may begin to disintegrate and break into fine pieces due to vibration.
The present invention has been made in view of the above situation. It aims at providing a honeycomb structure which resists breakage, and accordingly has superior durability and reliability, by reducing the thermal stress generated therein during use by a sharp temperature change of inflow gas, a local heat of reaction and a local heat of combustion.